Improve show-hint to customize icon, label, picked item in the completion + add sample with XML Hint.

Hi Marijnh,

Here a first patch to improve the show-hint to customize : 

 * icon, label in the completion (see xml sample which displays an icon per node type (element, attribute).
 *  picked item (see xml sample whith attribute. When you pick an attribute it add teh attribute name and '=""' and set the cursor on the middle of the quote.

I'm using this feature on my XQuery Hint (I will commit that after). I have modified the XML Hint to use this new feature (just override updateItem and pickedItem functions).

I have too move the xml hint in the addon/hint/xml folder. I think it should be cleaner if hint are classified by mode.